Motion sickness increases the risk of accidental hypothermia

Motion sickness (MS) has been found to increase body-core cooling during immersion in 28°C water, an effect ascribed to attenuation of the cold-induced peripheral vasoconstriction (Mekjavic et al. in J Physiol 535(2):619–623, 2001). The present study tested the hypothesis that a more profound cold stimulus would override the MS effect on peripheral vasoconstriction and hence on the core cooling rate. Eleven healthy subjects underwent two separate head-out immersions in 15°C water. In the control trial (CN), subjects were immersed after baseline measurements. In the MS-trial, subjects were rendered motion sick prior to immersion, by using a rotating chair in combination with a regimen of standardized head movements. During immersion in the MS-trial, subjects were exposed to an optokinetic stimulus (rotating drum). At 5-min intervals subjects rated their temperature perception, thermal comfort and MS discomfort. During immersion mean skin temperature, rectal temperature, the difference in temperature between the non-immersed right forearm and 3rd finger of the right hand (ΔT _ff), oxygen uptake and heart rate were recorded. In the MS-trial, rectal temperature decreased substantially faster (33%, P < 0.01). Also, the ΔT _ff response, an index of peripheral vasomotor tone, as well as the oxygen uptake, indicative of the shivering response, were significantly attenuated (P < 0.01 and P < 0.001, respectively) by MS. Thus, MS may predispose individuals to hypothermia by enhancing heat loss and attenuating heat production. This might have significant implications for survival in maritime accidents.     

The effect of postural changes on body temperatures and heat balance

Early studies have demonstrated that rectal temperature (T _re) decreases and mean skin temperature (T _sk) increases in subjects changing their posture from standing to supine, and vice versa. Such changes have important implications insofar as thermal stress experiments are conducted and interpreted. However, the extent of these changes between steady-state conditions is not known. In addition, it is not known whether thermal balance is also affected by postural changes. To examine these questions, 11 healthy males were exposed to a thermoneutral air environment (28.2–28.5°C and 40% relative humidity) in various postures at rest. Body temperatures, heat losses, and metabolic rate were measured. Subjects wore shorts only and began in an upright posture (standing or sitting at an inclination of 7.5°) on a customized tilt-table. They were tilted twice, once into a supine position and then back to the original upright position. Each tilt occurred after steady state was satisfied based on the subject's circadian variation of T _re determined previously in a 4.25 h control supine trial. Times to supine steady state following the first tilt were [mean (SE)] 92.6 (6.4) and 116.6 (5.1) min for the standing and sitting trials, respectively. Times to upright steady state following the second tilt were 107.9 (11.4) and 124.1 (9.0) min. Mean steady-state T _re and T _sk were 36.87 (0.07) and 34.04 (0.14), 37.47 (0.09) and 33.48 (0.14), and 37.26 (0.05) and 33.49 (0.10) °C for supine, standing, and sitting, respectively. Thermal balance was attained in all steady-state conditions, and allowing for a decrease in the weighting factor of T _re for mean body temperature in the upright postures, it also appears that thermal balance was preserved between changes in posture. These results are consistent with no perceived changes by the subjects in their thermal comfort and skin wetness.     

The effects of warming by active and passive means on the subsequent responses to cold water immersion

Two experiments were undertaken to investigate the effects of warming the body upon the responses during a subsequent cold water immersion (CWI). In both experiments the subjects, wearing swimming costumes, undertook two 45-min CWIs in water at 15° C. In experiment 1, 12 subjects exercised on a cycle ergometer until their rectal temperatures (T _re) rose by an average of 0.73°C. They were then immediately immersed in the cold water. Before their other CWI they rested seated on a cycle ergometer (control condition). In experiment 2, 16 different subjects were immersed in a hot bath (40° C) until their T _re rose by an average of 0.9° C; they were then immediately immersed in the cold water. Before their other CWI they were immersed in thermoneutral water (35° C; control condition). Heart rate in both experiments and respiratory frequency in experiment 1 were significantly (P < 0.05) higher during the first 30 s of CWI following active warming. In experiment 1, the rate of fall of T _re during the final 15 min of CWI was significantly (P < 0.01) faster when CWI followed active warming (2.46° C · h^–1) compared with the control condition (1.68°C · h^–1). However, this rate was observed when absolute T _re was still above that seen in the control CWIs. It is possible, therefore, that if longer CWIs had been undertaken, the two temperature curves may have converged and thereafter fallen at similar rates; this was the case with the aural temperature (T _au) seen in experiment 1 and the T _au and T _re in experiment 2. It is concluded that pre-warming is neither beneficial nor detrimental to survival prospects during a subsequent CWI.     

Thermoregulation during exercise in relation to sex and age

Summary The thermoregulatory responses to 1 h exercise of 14 male (age range 18–65 year) and 7 female (age range 18–46 year) athletes and 4 (3 and 1 ) non-athletic subjects have been investigated in a moderate environment (T _db=21 C, T _wb=15 C and rh<50%) and analysed in relation to age, sex, and maximum aerobic power output (VO₂ max).The maximal sweat loss (M _{sw max}) under the given conditions was closely related (r=+0.90) to VO_{2 max} and for a given relative work load (%VO_{2 max}), rectal (T _re) and mean skin (¯T _sk) temperatures was the same in all subjects.Sweat loss (004d _sw) was linearly related to total heat production (H) and to peripheral tissue heat conductance (K) and if expressed in relative terms (%M _{sw max}) was linearly related to T _re. For a given T _re relative sweat rate was identical in the groups studied. From these results it would seem that during exercise T _re rises to meet the requirements of heat dissipation by establishing a thermal gradient from core to skin and stimulating sweating in proportion to maximal capacity of the system. Thus provided the thermal responses to work were standardised using the appropriate physiological variables, there was no evidence to be found for differences in thermoregulatory function which could be ascribed to sex or age.     

Thermoregulation during heat exposure of young children compared to their mothers

The study was conducted to investigate the thermoregulation of young children compared to that of adults. A group of 19 children (ages 9 months-4.5 years), with only 3 children aged 3 years or above, and 16 adults first rested in a thermoneutral room (air temperature 25°C relative humidity 50%, air velocity 0.2 m·s^–1). They were then exposed to a hot room (air temperature 35°C, relative humidity 70%, air velocity 0.3 m·s^–) next door for 30 min, and then returned to the thermoneutral room where they stayed for a further 30 min. The rectal temperature (T _re), skin temperatures (T _sk) at seven sites, heart rate (HR), total sweat rate ( ), local sweat rate ( ) and the Na⁺ concentration of the sweat were measured. There was no significant difference inT _re between the children and their mothers in the rest phase. However, theT _re of the children increased as soon as they entered the hot room and was significantly higher than during the control period, and than that of the mothers during heat exposure. MeanT _sk, forehead, abdomen and instepT _sk were significantly higher in the children during both the thermoneutral and heat exposure. The was significantly higher and Na⁺ concentrations in the sweat on the back and upperarm were significantly lower for the children during the heat exposure. They had a greater body surface area-to-mass ratio than the mothers by 64%, which indicated that they had advantages for thermal regulation. However, the sweating andT _sk responses of the children were not enough to prevent a rise in body temperature. These results would suggest that the young children had the disadvantage of heating up easily due to their smaller body sizes and there may be maturation-related differences in thermoregulation during the heat exposure between young children and mothers.     

Responses of young and older men during prolonged exercise in dry and humid heat

Summary Eight young, sedentary men (aged 34 years, SD 3) and six older moderately active, unacclimated men (aged 57 years, SD 2) walked on a treadmill at 30% of their maximum oxygen consumption up to 3.5 h in a thermoneutral [dry bulb temperature (T _db) 21°C, relative humidity (r.h.) 43%)], a warm humid (T _db 30°C, r.h. 80%) and a hot dry (T _db 40°C, r.h. 20%) environment while wearing ordinary working clothes (0.7 c/o). Their oxgen consumption, heart rate (f _c), rectal (T _re) and mean skin temperature (T_sk), sweat rate (SR), and evaporative rate (ER) were measured during the tests. The ratings of thermal sensation (TS) and perceived exertion (RPE) were assessed using standard scales. In the heat stress tests, the number of experiments discontinued did not significantly differ between the two groups. The mean levels and end-exercise values of T _re, T_sk, f _c, TS and RPE were not significantly different between the young and older subjects in any of the environments. In the warm humid environment, however, the T _re and RPE of the older subjects increased continuously (P<0.05) during the test compared to the young subjects. No significant difference between the groups was observed in SR or in ER. In the hot dry environment, however, the ER of older men increased more slowly compared to the young men. In spite of some time-related differences observed in T _re, RPE, and ER, the older subjects did not exhibit higher f _c during exercise in the heat, they were not more hyperthermic and their performance times were similar to the young subjects. Therefore, it was concluded that older calendar age is not necessarily associated with a reduced ability to exercise in a hot environment and other factors, such as physical activity habits and aerobic capacity, may be equally important in determining heat tolerance in the elderly.     

Influence of the menstrual cycle and oral contraceptives on thermoregulatory responses to exercise in young women

Summary Thermoregulatory responses to exercise in relation to the phase of the menstrual cycle were studied in ten women taking oral contraceptives (P) and in ten women not taking oral contraceptives (NP). Each subject was tested for maximal aerobic capacity ( ) and for 50% exercise in the follicular (F) and luteal (L) phases of the menstrual cycle. Since the oral contraceptives would have prevented ovulation a quasi-follicular phase (q-F) and a quasi-luteal phase (q-L) of the menstrual cycle were assumed for P subjects. Exercise was performed on a cycle ergometer at an ambient temperature of 24° C and relative air humidity of 50%. Rectal (T _re), mean skin ( ), mean body ( ) temperatures and heart rate (f _c) were measured. Sweat rate was estimated by the continuous measurement of relative humidity of air in a ventilated capsule placed on the chest, converted to absolute pressure (PH₂O_chest). Gain for sweating was calculated as a ratio of increase inPH₂O_chest to the appropriate increase inT _re for the whole period of sweating (G) and for unsteady-state (G_u) separately. The did not differ either between the groups of subjects or between the phases of the menstrual cycle. In P, rectal temperature threshold for sweating (T _{re, td}) was 37.85° C in q-L and 37.60° C in q-F (P < 0.01) and corresponded to a significant difference fromT _re at rest. TheT _re, andf _c increased similarly during exercise in q-F and q-L. No menstrual phase-related differences were observed either in the dynamics of sweating or in G. In NP,T _{re, td} was shorter in L than in F (37.70 vs 37.47° C,P<0.02) with a significantly greater value fromT _re at rest. The dynamics and G for sweating were also greater in L than in F. The G_u was 36.8 versus 16.6 kPa · ° C^–1 (P<0.01) while G was 6.4 versus 3.8 kPa · ° C^–1 (P<0.05), respectively. TheT _re, andf _c increased significantly more in phase F than in phase L. It was concluded that in these women performing moderate exercise, there was a greater temperature threshold and larger gains for sweating in phase L than in phase F. Intake of oral contraceptives reduced the differences in the gains for sweating making the thermoregulatory responses to exercise more uniform.     

Effects of Atropine and Β-blockade on temperature regulation and performance during prolonged exercise

Summary The effects of intravenous injections of Atropine (1.8 mg) and practolol (15 mg) on the thermoregulatory responses to 1 h of exercise on a motordriven treadmill have been investigated on six healthy subjects.The results show that -blockade had little effect on thermal responses to work except for a small but significant (p<0.05) decrease in mean skin temperature (¯T _sk ) and peripheral tissue heat conductance (K). Metabolic (M) and total heat (H) production, and evaporative sweat loss (E) and rectal temperature (T _re ) were similar to control values. In contrast, atropine, particularly at work loads beyond 60% maximal aerobic power output (VO_{2 max}), raised T _re (p<0.001), ¯T _sk (p<0.001) and reduced E by approximately 50%. At the highest work loads T _re increased as a linear function of time during the latter part of exercise, and at the 60th min was almost independent of relative stress (expressed as % VO_{2 max}) imposed on the subjects. At the lower work loads the majority of subjects reached thermal equilibrium before the end of exercise by maintaining their convective heat transfer from core to periphery by increasing peripheral blood flow (as indicated by K), and raising their heat losses to environment by convection and radiation. The latter pathways for heat dissipation were enhanced by the subjects ability to sustain a ¯T _sk 4 C above control values independently of M. Atropine had no effect on M or H but greatly affected work performance, no subject was able to exercise at loads >70% VO_{2 max} for 1 h. These results demonstrate the ability of the thermoregulatory system to adapt to -adrenergic and to parasympathetic blockade during light exercise, and underline the effects of a reduction in the capacity of the sweating mechanism on physiological performance at higher rates of work.List of Abbreviations used in the Text M Metabolic heat production - H Total heat production - E Evaporative sweat loss - T _re Rectal temperature - ¯T _sk Mean skin temperature - K Peripheral tissue heat conductance - PBF Peripheral blood flow - VO_{2 max} Maximal aerobic power output - f _H Cardiac frequency     

The effects of pre-warming on the metabolic and thermoregulatory responses to prolonged submaximal exercise in moderate ambient temperatures

To determine the effects of pre-warming on the human metabolic and thermoregulatory responses to prolonged steady-rate exercise in moderate ambient temperatures and relative humidities [means (SD) 21.7 (2.1)° C and 36.7 (5.4)%, respectively], six healthy men each ran at a steady-rate (70% maximal oxygen uptake) on a treadmill until exhausted after being actively pre-warmed (AH), passively pre-warmed (PH), and rested (Cont). Exercise time to exhaustion was significantly reduced following both AH and PH compared to Cont [AH 47.8 (14.0) min, PH 39.6 (16.0) min, Cont 62.0 (8.8) min; P<0.05]. During exercise there were no significant differences in oxygen uptake, total sweat loss, mean skin temperature (T_sk) and the thermal gradient (T _re–T_sk, where T _re is rectal temperature) following the three conditions. Serum prolactin, plasma catecholamine and plasma free fatty acid concentrations were also similar between all three trials. In contrast, T _re, mean body temperature, heart rate and ratings of perceived exertion were significantly greater during the initial 25 min of exercise following both AH and PH, compared with Cont (P<0.05). At exhaustion, there were no significant differences in the metabolic and thermoregulatory responses to exercise between the trials. The current findings demonstrate that AH and PH promote a reduction in prolonged submaximal endurance performance under moderate environmental temperatures compared with pre-exercise rest. Such observations appear likely to have been mediated through mechanisms associated with the earlier development of high internal body temperature which resulted in changes in the capacity for heat storage. Electronic Publication     

Seasonal changes in circadian rhythms of body temperatures in humans living in a dry tropical climate

Summary Seven volunteers (3 females and 4 males; 3 Caucasians and 4 Africans) participated in two 24 h sessions during the cool dry (CD) and the hot dry (HD) seasons of the sahelian tropical climate. Body temperatures were taken on portable cassette recorders for 24 h. Rectal (T _re) and mean skin (¯T _sk) temperatures decreased in the HD compared to the CD conditions, meeting one of the criteria for adaptation to heat. No ethnic differences in thermal responses were found. Males and females differed in their body temperature rhythms and in their reactions to heat. Body temperatures were higher in females than in males. Males reacted to heat with a decrease in T _re, without change in the T _re-¯T _sk gradient. Females showed a decrease in both T _re and ¯T _sk, more marked for ¯T _sk, with an increase in the T _re-¯T _sk gradient. It was concluded that males showed seasonal acclimatization to heat via a decrease in metabolism confirmed by a decrease in plasma levels of thyroid stimulating hormone (TSH) in the HD condition. Females showed a mixed metabolic and thermolytic type of acclimatization, with an absence of variation in plasma TSH levels. In conclusion, the steady rise in temperature between the CD and HD conditions was sufficient to trigger an acclimatization to heat similar in Caucasian and African subjects, although exposure to the external climate differed widely.     

Dynamics of sweating in men and women during passive heating

Summary The dynamics of sweating was investigated at rest in 8 men and 8 women. Electrical skin resistance (ESR), rectal temperature (T_re) and mean skin temperature were measured in subjects exposed to 40‡ C environmental temperature, 30% relative air humidity, and 1 m · s⁻¹ air flow. Sweat rate was computed from continuous measurement of the whole body weight loss. It was found that increases in T_re, and mean body temperature were higher in women than in men by 0.16, 0.38 and 0.21‡ C, but only the difference in δ was significant (p<0.05). The dynamics of sweating in men and women respectively, was as follows: delay (t_d) 7.8 and 18.1 min (p<0.01), time constant (Τ) 7.5 and 8.8 min (N.S.), inertia time (t_i) 15.3 and 26.9 min (p<0.002), and total body weight loss 153 and 111 g · m⁻² · h⁻¹ (p<0.001). Dynamic parameters of ESR did not differ significantly between men and women. Inertia times of ESR and sweat rate correlated in men (r=0.93, p<0.001), and in women (r=0.76, p<0.02). In men, δ T_re correlated with inertia time of sweat rate (r=0.81, p<0.01) as well as with the inertia time of ESR (r=0.83, p<0.001). No relation was found between δ T_re and the dynamics of sweating in women. It is concluded that the dynamics of sweating plays a decisive role in limiting δ T_re in men under dry heat exposure. The later onset of sweating in women does not influence the rectal temperature increase significantly. In women, δ T_re is probably limited by a complex interaction of sweating, skin blood flow increase, and metabolic rate decrease. This work was supported by the Centre National de la Recherche Scientifique and Polish Academy of Siences     

Thermoregulation in hyperhydrated men during physical exercise

Summary The influence of hyperhydration on thermoregulatory function was tested in 8 male volunteers. The subjects performed cycle exercise in the upright position at 52%V _{O 2max} for 45 min in a thermoneutral (Ta=23° C) environment. The day after the control exercise the subjects were hyperhydrated with tap water (35 ml · kg^–1 of body weight) and then performed the same physical exercise as before.Total body weight loss was lower after hyperhydration (329±85 g) than during the control exercise (442±132 g),p<0.05. The decrease in weight loss after hyperhydration was probably due to a decrease in dripped sweat (58±64 and 157±101 g,p<0.05). With hyperhydration delay in onset of sweating was reduced from 5.8±3.2 to 3.7±2.0 min (p<0.05), and rectal temperature increased less (0.80±0.20 and 0.60±0.10° C,p<0.01). The efficiency of sweating was higher in hyperhydrated (81.4%) than in euhydrated subjects (57.1%),p<0,01. It is concluded that hyperhydration influences thermoregulatory function in exercising men by shortening the delay in onset of sweating and by decreasing the quantity of dripped sweat. As a result, the increases in body temperature in hyperhydrated exercising men are lower than in normally hydrated individuals.     

Comparison in men of physiological responses to exercise of increasing intensity at low and moderate ambient temperatures

Summary In six male subjects the sweating thresholds, heart rate (f _c, as well as the metabolic responses to exercise of different intensities [40%, 60% and 80% maximal oxygen uptake (VO_2max)], were compared at ambient temperatures (T _a) of 5° C (LT) and 24° C (MT). Each period of exercise was preceded by a rest period at the same temperature. In LT experiments, the subjects rested until shivering occurred and in MT experiments the rest period was made to be of exactly equivalent length. Oxygen uptake (VO₂) at the end of each rest period was higher in LT than MT (P< 0.05). During 20-min exercise at 40%VO_2max performed in the cold no sweating was recorded, while at higher exercise intensities sweating occurred at similar rectal temperatures (T _re) but at lower mean skin (T _sk) and mean body temperatures (T _b) in LT than MT experiments (P<0.001). The exercise inducedVO₂ increase was greater only at the end of the light (40%VO_2max) exercise in the cold in comparison with MT (P<0.001). Bothf _c and blood lactate concentration [la]_b were lower at the end of LT than MT for moderate (60%VO_2max) and heavy (80%VO_2max) exercises. It was concluded that the sweating threshold during exercise in the cold environment had shifted towards lower (T _b) andT _sk. It was also found that subjects exposed to cold possessed a potentially greater ability to exercise at moderate and high intensities than those at 24° C since the increases inT _re,f _c and [la]_b were lower at the lowerT _a.     

Modification of internal temperature regulation for cutaneous vasodilation and sweating by bright light exposure at night

Bright light (BL) exposure at night leads to suppressed secretion of melatonin and an attenuated fall in internal temperature at rest from the night to the early morning. However, it is unknown at the present whether typical diurnal variations in reflex responses to thermal challenges are similarly affected by BL exposure at night. We investigated the control of cutaneous vasodilator and sweating responses to hyperthermia in the early morning after artificial BL exposure at night, compare with dim light (DL) exposure. Six subjects stayed awake in a semi-supine position under DL (120 lx) or BL (2800 lx) conditions between 21.00 and 04.30 h. Urine samples were collected at 04.30 h. Beginning at 05.30 h, the lower legs were immersed for 50 min in 42°C water. The subjects remained awake for 21 h until the end of hot water immersion. Urinary 6-sulphatoxymelatonin levels following BL were significantly lower than after DL. Oesophageal temperature (T_es) before heating was significantly higher following BL [36.41±0.10 (DL) vs. 36.55±0.09 (BL)°C]. The T_es thresholds for the onset of cutaneous vasodilation and sweating were significantly higher with BL than with DL conditions (approximately 0.15°C, respectively). We found that the internal temperature threshold for thermoregulatory control of cutaneous vasodilation and sweating responses to passive heating in the early morning can be modified by the level of light exposure the prior night. Thus both basal internal temperature and the regulation of internal temperature are modified by BL exposure at night.     

Differences in regional sweating responses during exercise between athletes trained on land and in water

We investigated whether there are any differences in regional sweating responses during exercise between athletes trained on land and in water. We measured the local sweating rates on the left forearm (m_swf) and the left scapula (m_sws), body temperatures (mean skin temperature, and rectal temperature T _re) in eight athletes trained on land (five soccer players, one distance runner and two baseball players, L group) and seven athletes trained in water (seven swimmers, W group) during cycle ergometer exercise at 50% maximal oxygen uptake for 40 min. The heart rate and oxygen uptake in the two groups during exercise showed nearly the same pattern of change. The T _re at the end of the exercise were 38.13 (SEM 0.19)°C in the L group and 38.26 (SEM 0.34)°C in the W group. Although the m_swf in the two groups were similar, m_sws were significantly higher in L than in W at 30, 35 and 40 min of exercise. The m_sws at any given mean body temperature tended to be greater in L than in W. These results showed that a difference in regional sweating rate during exercise between the athletes trained on land and in water was present on the scapula.     

System properties, feedback control and effector coordination of human temperature regulation

The aim of human temperature regulation is to protect body processes by establishing a relative constancy of deep body temperature (regulated variable), in spite of external and internal influences on it. This is basically achieved by a distributed multi-sensor, multi-processor, multi-effector proportional feedback control system. The paper explains why proportional control implies inherent deviations of the regulated variable from the value in the thermoneutral zone. The concept of feedback of the thermal state of the body, conveniently represented by a high-weighted core temperature (T _c) and low-weighted peripheral temperatures (T _s) is equivalent to the control concept of “auxiliary feedback control”, using a main (regulated) variable (T _c), supported by an auxiliary variable (T _s). This concept implies neither regulation of T _s nor feedforward control. Steady-states result in the closed control-loop, when the open-loop properties of the (heat transfer) process are compatible with those of the thermoregulatory processors. They are called operating points or balance points and are achieved due to the inherent property of dynamical stability of the thermoregulatory feedback loop. No set-point and no comparison of signals (e.g. actual-set value) are necessary. Metabolic heat production and sweat production, though receiving the same information about the thermal state of the body, are independent effectors with different thresholds and gains. Coordination between one of these effectors and the vasomotor effector is achieved by the fact that changes in the (heat transfer) process evoked by vasomotor control are taken into account by the metabolic/sweat processor.     

Thermally induced salivary secretion in anesthetized rats

Effect of thermal stimulation on salivary secretion was examined in urethane anesthetized (i.p., 1.0 g/kg) rats. First, rectal temperature (T _re) was maintained at various levels by warming the whole trunk with a heating blanket. Copious salivary secretion occurred whenT _re reached a threshold value above 40°C, which is considerably higher than the threshold for tail vasodilation. Local warming of the scrotum, face, or hypothalamus also elicited salivary secretion, but only ifT _re was in a limited range just below the threshold temperature at whichT _re alone would induce salivary secretion. The higher theT _re within that limited range, the lower the temperature of the site locally warmed at which salivary secretion began. Changes in temperature of the abdomen, not including the scrotum, modulated the salivary secretion elicited by scrotal warming. Hypothalamic and scrotal temperatures interacted with each other to affect salivary secretion. Temperature signals from both core and periphery thus appear to be integrated in bringing about salivary secretion. Thermally induced salivary secretion may function as a basis for saliva spreading behavior observed in rats in a hot environment.     

Seasonal variation in sweating responses of older and younger men

Eight older (60–65 years) and six younger (20–25 years) men were exposed to a standard heat stress for 60 min in summer, autumn, winter, and spring. The test consisted of placing the lower legs and feet in a 42°C water bath while sitting in constant environmental conditions (30°C and 45% relative humidity). The increase of rectal temperature (T _re) was significantly greater (P < 0.05) in autumn, winter, and spring than in summer for the older group, but significantly greater only in winter than in summer for the younger group (P < 0.05). The T _re was greater for the older group in all seasons, but of significance only in autumn and spring (P < 0.01). There were no significant season-related differences for metabolic heat production (m) and mean skin temperature ( _sk) during the heat test in the respective groups, although the m and _sk were lower for the older group in all seasons (P < 0.01). In the older group total body sweating rate (m_sw) divided by T _re (total m_sw/T _re) decreased from summer to winter (P < 0.02) and did not differ between winter and spring, whereas total m_sw/T _re in the younger group increased in spring after decreasing from autumn to winter (P < 0.03). The variations of the value, local sweating rate on the back and thigh divided by T _re (back m_sw/T _re and thigh m_sw/T _re), were similar to those of the total m_sw/T _re in each group, except for back m_sw/T _re in the younger group, which did not increase from winter to spring. The total m_sw/T _re, back m_sw/T _re and thigh m_sw/T _re were significantly less for the older group in summer, autumn and spring (P < 0.05). The range of seasonal variations was significantly less for the older group (P < 0.001). The results indicated that, compared with younger men in older men, the enhancement of sweating function toward summer occurred later and its reduction toward winter occurred earlier despite a smaller range of seasonal variation and that older men had a somewhat lesser capability to maintainT _re when challenged by heat stress in all seasons.     

Thermal changes observed before and after J.-L. Etienne's journey to the North Pole

Summary The thermoregulatory responses of a French doctor, Jean-Louis Etienne, were examined in a standard cold test before and after his journey to the North Pole, to investigate whether general and/or local cold adaptation had occurred. The two tests were carried out in a climatic chamber for 2 h at rest (dry bulb temperature, 1° C; relative humidity, 40%; wind speed, 0.8 m · s^–1). After his journey, Etienne showed a general hypothermic-hypometabolic adaptation, i.e. a decrease in rectal temperature (T _re) and metabolic heat production (M), and an increased local skin temperature of the extremities. Between the two tests, a change occurred in the relationship between tympanic temperature (T _ty) and M. During the post-journey cold test, T _ty [as representative of the central nervous system (CNS) temperature] increased while the decrease in T _re was accelerated, probably due to a redistribution of blood volume towards the CNS. Such a mechanism would protect the central core with special reference to the CNS.     

Thermal responses of men and women during cold-water immersion: influence of exercise intensity

Summary The influence of exercise intensity on thermoregulation was studied in 8 men and 8 women volunteers during three levels of arm-leg exercise (level I: 700 ml oxygen (O₂) · min^–1; level II: 1250 ml O₂ · min^–1; level III: 1700 ml O₂ · min^–1 for 1 h in water at 20 and 28°C (T _w). For the men inT _w 28°C the rectal temperature (T _re) fell 0.79°C (P<0.05) during immersion in both rest and level-I exercise. With level-II exercise a drop inT _re of 0.54° C (P < 0.05) was noted, while at level-III exerciseT _re did not change from the pre-immersion value. AtT _w of 20°C,T _re fell throughout immersion with no significant difference in finalT _re observed between rest and any exercise level. For the women at rest atT _w 28°C,T _re fell 0.80°C (P<0.05) below the pre-immersion value. With the two more intense levels of exercise,T _re did not decrease during immersion. InT _w 20°C, the women maintained higherT _re (P<0.05) during level-II and level-III exercise compared to rest and exercise at level I. The_{T re} responses were related to changes in tissue insulation (I _t) between rest and exercise with the largest reductions inI _t noted between rest and level-I exercise acrossT _w and gender. For men and women of similar percentage body fat, decreases inT _re were greater for the women at rest and level-I exercise inT _w 20°C (P< 0.05). With more intense exercise, the women maintained a higherT _re than the men, especially in the colder water. These findings indicate that exercise is not always effective in offsetting the decrease inI _t and facilitated heat loss in cool or cold water compared to rest. The factors of exercise intensity,T _W, body fat, and gender influence the thermoregulatory responses.